Today I went to a local Asian supermarket (Uwajimaya, if you're in PDX) and bought a big ol' piece of bamboo, the size of my fist. I think it's peeled already because it's all one color and was stored in water. Maybe it's precooked? I don't know. Does anyone know how to cook it?

Yes the pale one in a bag is ready to use. The way I learned to cook it is to cut it into quarters, then cut each quarter into little slices. Then mix equal parts white miso and sake in a bowl (like 2 spoons of each), stir fry some red chilli in sesame oil then add the bamboo slices and cook a bit more just to blend flavors. Reduce the heat and add the miso/sake mixture to pan, toss to coat.

I've never seen a chunk right out of the ground for sale, but I know it spoils quickly so if that is what you have, get it cooked as soon as you can.